Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-12-04
2004-11-16
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S666000
Reexamination Certificate
active
06818080
ABSTRACT:
DESCRIPTION
This application is a Continuation of International Application No. PCT/JP01/03028, filed Apr. 9, 2001.
FIELD OF THE INVENTION
The present invention relates to a manufacturing method for shaped light metal article where an article for plastic working of light metal is plastic worked and the resulting plastic worked article is heat treated.
BACKGROUND OF THE INVENTION
One method of shaping metal materials is the plastic working method called “forging”. Forging is where a metal material, such as a billet, is set in a die and is hammered into a desired shape. When forging a light metal material, it is customary to subject the forged articles produced by forging to a T6 heat treatment to improve the mechanical properties. A T6 heat treatment is a two-step heat treatment composed of a solution treatment, where a high temperature is maintained for a predetermined time to increase the homogeneity of a material composition, and subsequently an ageing precipitation hardening treatment, where a comparatively low temperature is maintained for a predetermined time to increase hardness.
Cast-forging, where casting and forging are combined, is another method for shaping a light metal material. Cast-forging is where casting is performed, such as by injection molding or die casting, to produce an article for forging in a shape that is close to the intended form, with the article for forging then being forged to work the article into the intended form Japanese Laid-Open Patent Publication H11-104800 (which corresponds to European Patent Publication: EP0905266 A1) discloses a method where forged article that has been shaped using cast-forging, which is made of a light metal material, is subjected to a T6 treatment composed of a solution treatment with a processing temperature in a range of 380 to 420° C. and a processing time in a range of 10 to 24 hours and an ageing precipitation hardening treatment with a processing temperature in a range of 170 to 230° C. and a processing time in a range of 4 to 16 hours.
However, when injection molding or die casting is used as the casting method performed during cast-forging, internal defects, such as gas defects, are produced in the article for forging. The number of such internal defects can be reduced, such as by having semimolten metal flow into the cavity or by improving the die, but it is extremely difficult to completely eradicate such internal defects. When article for forging include internal defects, there are the problems that performing a standard T6 heat treatment after forging does not sufficiently improve the mechanical characteristics and that the appearance of the forged article is spoilt by the creation of swelling-like blisters on their surface due to the expansion of gas defects during heat treatment.
The above problems can be solved by performing a pre-forging heat treatment with the aims of converting the article for forging to a solution and expanding the gas defects, and, after the heat-treated article for forging have been forged, a post-forging heat treatment with the aim of improving the mechanical properties. With this method, the forging process ruptures and eradicates some of the blisters that appear in the surface of the article for forging due to the expansion of gas defects during the pre-forging heat treatment, resulting in a reduction in the number of gas defects present in the forged article.
However, the post-forging heat treatment is performed under the same conditions as the ageing precipitation hardening treatment that forms part of the T6 treatment. This results in the problem of the shaped light metal article produced by this method having poor ductility.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a manufacturing method which optimizes the conditions for heat treatment performed on a plastic worked article after plastic working and so produces shaped light metal article with sufficient ductility.
In order to achieve the stated object, the present invention subjects a plastic worked article made of light metal material to a post-plastic working heat treatment that has a higher temperature and shorter processing time than the ageing precipitation hardening treatment performed in a standard T6 treatment.
In more detail, the present invention is a method of manufacturing a shaped light metal article, including the steps of forming a plastic worked article by plastic working an article for plastic working made of light metal material; and subjecting the plastic worked article to a post-plastic working heat treatment at a temperature in a range of 250 to 400° C. for between 20 minutes and 10 hours.
With the above method, a post-plastic working heat treatment that has a higher temperature and shorter processing time than the ageing precipitation hardening treatment of a T6 treatment is performed. As can be understood from the experiments described later in this specification, this enables ductility to be effectively improved, while maintaining the strength and yield strength.
A temperature range of 250 to 400° C. is used since a sufficient improvement in ductility cannot be achieved at temperatures below 250° C. and a significant decrease in yield strength occurs at temperatures above 400° C.
A processing time in a range of 20 minutes to 10 hours is used since a sufficient improvement in ductility cannot be achieved by processing for less than 20 minutes and there are cases where heat treatment for more than 10 hours results in a decrease in ductility. The processing time preferably is set at 5 hours or shorter, with 1 hour being optimal.
The expression “light metal material” refers to a metal, such as aluminum or magnesium, with a low density, or to an alloy of such. One specific example is AZ91D under ASTM Standards.
Plastic working here refers to forging or the like.
Even when the present kind of post-plastic working heat treatment is performed, the presence of a large number of internal defects such as gas defects in the plastic worked article subjected to this heat treatment stops the above effects from being sufficiently obtained.
In case that the light metal material is formed of light metal alloy, if the article for plastic working is subjected to a pre-plastic working heat treatment that uses a temperature that is lower than a temperature at which eutectic of the light metal alloy starts to be fused, blisters can be produced in the surface of the article for plastic working due to the expansion of gas defects included near the surface of the article for plastic working. Some of these blisters are ruptured and eradicated during the plastic working, thereby reducing the number of gas included defects in the plastic worked article. The reason that the heat treatment is performed at the temperature lower than a temperature at which eutectic of the light metal alloy starts to be fused is that at a temperature equal to or higher than the temperature, the article for plastic working is partially fused and the material composition of the fused part is not homogenized, which involves a break from the fused part at the plastic working. It is preferable for the processing temperature to be in a range of 350° C. to 450° C. As blisters are created before plastic working and are eradicated by the plastic working, the further creation of blisters by the post-plastic working heat treatment can be suppressed, resulting in a favorable appearance for the shaped light metal article produced by this method.
When the processing time of the pre-plastic working heat treatment is one hour or longer, blisters can be effectively produced in the surface of the article for plastic working, and, in the same manner as the solution treatment performed in a T6 treatment, the homogeneity of the material composition can also be improved. For this reason, it is preferable for the processing time to be between 10 and 20 hours.
By making both the processing time and processing temperature of the pre-plastic working heat treatment respectively longer and higher than the processing time and processing tem
Sakamoto Kazuo
Sakate Nobuo
Uosaki Yasuo
Ip Sikyin
Mazda Motor Corporation
Nixon-Peabody, LLP
Studebaker Donald R.
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